Feature/add stratified tes

[MaGering]

With this PR the stratified thermal energy storage is added to the sector coupling model.

At present, in the mvs the oemof.solph component GenericStorage is defined with the following input parameters (cf. D1_model_components.py):

• label
• nominal_storage_capacity
• initial_storage_level
• loss_rate
• inflow_conversion_factor
• outflow_conversion_factor
• min_storage_level
• max_storage_level

To implement the oemof.thermal stratified thermal storage, two further input parameter would be needed (cf. example 01_operation_generic_storage.py):

• fixed_losses_relative
• fixed_losses_absolute

They are used to take into account temperature dependent losses of the GenericStorage.

The procedure within this PR is as follows:

• Implement basic structure containing "storage_02" as thermal energy storage with for now arbitrary input parameters
• Integrate pre-calculations of the stratified thermal storage from oemof.thermal into the model in a separate file called stratified_thermal_storage.py
• Write results of nominal_storage_capacity and loss_rate into storage_02.csv (installedCap and efficiency)
• Find a way to extend the model by two additional parameters from oemof.thermal (fixed_losses_relative and fixed_losses_absolute)
• Research and adapt input parameters of the stratified thermal storage

The following steps were realized, as well (if applies):

• Use in-line comments to explain your code
• Write docstrings to your code
• For new functionalities: Explain in readthedocs
• Write test(s) for your new patch of code
• ❌ Update the CHANGELOG.md (let's start this after the first release)
• Apply black (black . --exclude docs/)

Please mark above checkboxes as following:

• Open
• Done

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_{For more information on how to contribute check the CONTRIBUTING.md.}

[MaGering]

Thank you @MaGering! :)
I'm sorry, I still have some comments, but I think it's not so much work anymore!

No, I'm happy you have comments. Your review has helped me a lot, thank you! 🙏

* My overview over the directories we are using is not so good anymore, but I think `stratified_thermal_storage.csv` is missing in `data/user_inputs_collection/pvcompare_inputs/`.

Thanks for finding this! Added with commit 6a2e745.

[SabineHaas]

I made it through the tests now! Only a few changes then you can merge :)